PRE-CAMBRIAN ROCKS 443 



The common type of schist is colored green by the abundant chlorite. 

 Epidote occurs in the body of the rock as well as in conspicuous nodules 

 and veins. Some of these veins are a foot in width, but most of them 

 are less than half an inch wide. The epidotization has apparently taken 

 place in connection with the folding of rocks in pre-Cambrian times and 

 also in connection with later granitic intrusion. Epidote is often found 

 covering quartzite pebbles, as if replacing quartzite. One large "augen" 

 of feldspar has been found in the schist. 



Under the microscope the normal schist is seen to consist of chlorite, 

 sericite, quartz, magnetite, biotite, and muscovite. The rock is very fine- 

 grained. 



Hornblende schist is found in the vicinity of Sneech Pond and also a 

 mile north of Little Pond. It shows hornblende crystals half an inch 

 in length by a quarter of an inch in width. The origin of the rock is 

 possibly igneous. 



The blue quartz schist west of Manville occurs at the contact of the 

 Ashton schist with the Milford granite. The quartz crystals are rather 

 conspicuous in a matrix of greenish mica and fine quartz grains. In 

 thin-section the rock consists of knots of brecciatecl quartz, abundant bio- 

 tite, with smaller amounts of plagioclase, epidote, hornblende, chlorite, 

 and a little magnetite = In all probability this rock was of igneous origin. 



SMITHFIELD LIMESTONE 



Occasional patches of interbedded limestones occur in the Ashton 

 schist. Several beds of this Smithfield limestone occur 3 miles south of 

 Albion, at Lime Eock. Limestone was quarried on Copper Mine Hill in 

 the latter part of the eighteenth century for the copper it contained, and 

 one quarry is still visible about one-half mile east of Sneech Pond, north 

 of the road to Diamond Hill. 



Dr. C. T. Jackson in 1840^ reported a bed of limestone near Sneech 

 Pond 6 to 10 feet thick, with a strike of north 25 degrees west and a dip 

 of 35 degrees east and overlain by granite. He states that the limestone 

 contained chalcopyrite, tremolite, asbestos, and actinolite. The origin 

 of this limestone may be a large calcite vein in the Ashton schist, for sev- 

 eral smaller calcite veins occur in the vicinity. 



AQE RELATIONS 



Previous to the work of Emerson and Perry, the Blackstone series 

 were considered pre-Cambrian by C. T. Jackson, Crosby, Shaler, and 



» Report on the Geological and Agricultnral Survey of the State of Rhode Island. 



