100 HISTORICAL SKETCH

in the western part of the area to the highly crystalline rocks farther
east and to discover the nature of the transitions by which this crystal-
line structure appears to be progressively developed. Second, to map
minutely the highly crystalline rocks to the eastward, particularly
about Baltimore, and to work out in detail certain petrographic prob-
lems with reference to the eruptive masses which occur there in such
abundance. Considerable progress was made along these two lines,
and a brief statement of the results is given in the Tenth Annual Re-
port of the United States Geological Survey. ¹ These and other results
are also given in further detail in various unofficial publications.

Dr. Williams's operations in 1889-90 consisted of mapping the
areal distribution of the crystalline rocks and collecting material for
laboratory investigations in Harford, Baltimore, Carroll, Frederick and
Montgomery counties, Maryland, as well as in the District of Colum-
bia and in Fairfax county, Virginia.

Much progress was made in mapping the crystalline rocks of the
Washington and Baltimore quadrangles. Incidentally to the work on
the Baltimore quadrangle, a detailed investigation was made of a rare
type of eruptive rock composed wholly of pyroxene, and its alteration
into masses of steatite.

In the Frederick quadrangle the Triassic boundary was partially
traced; the region about Barnesville, especially about Sugar Loaf
Mountain, was mapped and studied; the great trap dike was traced
entirely across the state, and many areas of sandstones and limestones
were outlined. Incidentally a study was made of the Sykesville
granite and its inclusions. Some account of the results of these inves-
tigations was communicated to the Geological Society of America at
its Washington meeting, December, 1890, in a paper entitled the
" Petrography and Structure of the Piedmont Plateau in Maryland. "²

In 1890-91 the work was continued in Maryland and quite widely
into the adjoining Piedmont area of Virginia. The boundaries of the
Triassic in Maryland as determined by Professor Williams this season

¹ Tenth Ann. Rept. U. S. Geol. Survey, part i, 1890, pp. 152-154.
² Bull. Geol. Soc. America, March, 1891, vol. ii, pp. 301-318.