WHITE : PETROGRAPHY OF THE BOSTON BASIN. 
123 
granite these are found undecomposed and effervesce freely with 
acid. 
Most peculiar phenomena of rifting also occur along the walls of 
these cavities, as though they had been regions of special shrinkage 
and distortion. The same tendency is observed in thin sections, in 
the formation of minute rifts. In the Mill Cove area, limestone 
belts occur through the slates with accompanying epidotic, serpen- 
tinous or chloritic alteration products usually showing slickensides 
along the contacts. The microscope adds little information regard¬ 
ing these phenomena, as the products formed are so opaque, 
decomposed, and earthy that only dust of varying degrees of 
translucency is apparent, with scattered grains of quartz, chlorite, 
epidote, and limonite. The contact limestone evinces very thor¬ 
ough baking. Sears (’91) describes the same limestone from this 
locality and from near Cape Ann. 
The slate itself closely resembles that of the coast of Maine, as 
noted by Professor Slialer (’89). The slate of Eldridge Hill is very 
nearly the same as in the Paradoxides quarry, except that the 
dark grains are more scattered and the mottling is not so marked. 
The slate adjacent to the granite in the North Common Hill 
quarries is of considerably coarser texture and is less uniform. 
Microscopically it shows the compact texture which the German 
petrographers designate as “ hornfels,” or hornstone. 
The feldspar grains of this rock are often elongated in lath-shaped 
forms, intermingled with large flakes of partially altered and 
frayed-out biotite, some brown fibrous alteration product, and grains 
of magnetite. Single large-sized fragments of milky feldspars, 
partially colored by iron, are present throughout. 
In the same quarry, adjacent to the latter, is another type of 
hornfels of dusty, cryptocrystalline appearance. It contains abun¬ 
dant pink garnets, with very fresh and sharp outlines, varying from 
minute size up to one eighth of an inch. Tourmaline occurs in 
cavities throughout the hornfels in dark brown crystals. The rock 
is doubtless much baked as a consequence of its limited area in the 
midst of such a vast intrusion of igneous rock. 
In most of the localities the slates show microscopically only 
a fine dust of quartz, feldspar, iron ores, and dark silicates, some¬ 
times with larger frayed or indefinite scales of chlorite of various 
shades of color and degrees of alteration. There is nothing in 
the microscopic characters on which to base any correlation of the 
slates or distinguish the older from the newer. 
