MARYLAND. 69 
on 
This slate belt, whatever may be its exact bounds, is well exposed at Ijamsville, 
Bush Creek, and the Baltimore and Ohio Railroad in Frederick County, where it has 
a well-defined strike of N. 10° E. Between Ijamsville and a point 2\ miles south- 
southwestof that place it is at least 1£ miles wide, and reappears west of Hyattstown, 
3£ miles farther south in Montgomery County; and from that point its exposures and 
cleavage strike range from S 20° to 37° W. It thus passes between Sugarloaf .Moun- 
tain and the village of Mount Ephraim on the west and Hyattstown and Barnesville 
on the east, its minimum length and width being about 12 and 1^ miles respectively. 
Immediately east of this belt are chloritic schists and altered diabases. 
Ijamsville. — One-fourth mile west of Ijamsville station is an abandoned quarry in 
dark, slightly reddish purple slate. a The bedding, shown by light green ribbons, 
strikes N. 10°-15° E., and dips 30°-40° W., but the cleavage strikes N. 40° E. and 
dips 50° SE. There are intermittent joints striking N. 15°-20° E., dipping 75° EKE., 
which have been parted a fourth of an inch, lined with quartz and filled with cal- 
cite. Similar slate, but interbedded with light green, occurs also several hundred 
feet east and north of the station. 
Thurston. — On Little Bennett Creek, about a half mile east-southeast of Thurston, 
in Frederick County, is another quarry (opening 70 by 50 feet) operated twenty years 
ago but now abandoned. The slate is also dark purplish with light-green passages 
containing here and there a scaly bright-green mineral, pyrophyllite, which carries 
from 1 to 2 per cent of copper oxide. The bedding, indicated by coarsely plicated 
quartzose passages, appears to be about horizontal. A piece of this slate that is said 
to have been on a roof many years shows some lightening of the color, owing chiefly 
to the growth of lichens, but the change is only superficial. The cleavage strikes N. 
20° E. and dips eastward at an angle of 60°. There is a 2-foot quartz vein in the "top. ' ' 
About 600 feet south-southwest of this and one-half mile southeast of Thurston is the 
quarry recently opened or reopened by the Bennett Creek Slate Company, measur- 
ing 75 feet along the cleavage by 65 feet across it, which, in May, 1904. had a depth 
of 40 feet. The upper 25 feet are "top." The cleavage strikes N. 37° E. and dips 40° 
E., with green spots aligned in the same direction. Joints strike N. 30° E. and 
dip 40 ° W. The grain is almost at right angles to cleavage. The slate is dark 
purplish but has a bluer tinge than that of Ijamsville. To the unaided eye it has a 
slightly granular texture, a rather fine cleavage surface with distinct grain and a lit- 
|tle luster. It contains no magnetite, does not effervesce with cold dilute hydrochloric 
jicid, and is somewhat greasy to the touch. The sawn edges show light-green parti- 
pies and more minute pyrite ones. The sonorousness is medium. Some of the slates 
bn exposure develop dark spots, due to the oxidation of some. mineral. The amount 
,)f quartz is so small that slabs a half inch thick may be easily sawn across the cleav- 
ige and the grain with a handsaw. 
Under the microscope it shows brilliant aggregate polarization, but much irregu- 
arity in size of particles and very slight plications in the direction of the grain. 
Che matrix of muscovite (sericite) contains abundant quartz fragments, measuring 
p to 0.047 and sometimes 0.094 mm. There are conspicuous scales of chlorite, 
>ften interleaved with muscovite lying transverse to the cleavage, measuring up to 
.38 by 0.25 mm., exceptionally 0.66 by 0.14 mm., and forming the centers of lenses 
f sericite or talc? & measuring up to 1.5 mm. in length. There are also lenses of 
^ricite or talc? & measuring up to 0.56 by 0.09 mm. with their long axes in the grain 
irection. 
,«See Maryland Geol. Purvey, vol. 2, pp. 231, 232, pi. XXIV, fig. 2. 
&Mathews, following some determination by G. H. Williams, regarded these slates as containing 
klc. (Maryland Geol. Survey, Vol. 2, p. 232.) A chemical test just made by George Stei^er al the 
. S. Geol. Survey laboratory shows 0.27 percent of SiO->, soluble in 1-20 solution of Nu,s<> :; bn..r. 
Ignition, and 1.09 per cent after ignition. F. W. Clarke, chief chemist, notes that the difference, 0.82 
er cent, corresponds to a splitting off of SiG» from the talc if talc is present. It would be one-fourth 
2 \i the SiO- in the talc and thus represent a percentage of 5.17 of talc. As the only other silicates in 
ie slate are crystalline quartz, chlorite, muscovite (sericite), and tourmaline, this resull is reliable. 
he combination of chlorite and sericite alone would be sufficient to account for the "talcose 
uch of the slate. 
Bull. 275—06 6 
