ON CLEAVAGE AND FOLIATION IN ROCKS. 369 



and rapid motion. They were exceedingly tenacious of life, the cilia moving 

 until the water was dryed upon the glass. Some that I placed in a little 

 salt and water were alive the next day. The oysters on the table have been, 

 through the kindness of Mr. Sweeting, fishmonger, Cheapside, sent to me, 

 and are from one to four years old. 



It now, therefore, only remains to trace the life of the oyster and the 

 changes it undergoes from the state I in which found it in the parent until it 

 has formed its shell and attached itself to some substance, which I hope to be 

 able to do next year in a continuation of this Report. From the reports I 

 have received and my own observations, I think that the fence months might 

 be advantageously altered on many beds, and that if such alteration was 

 made, the markets might be supplied the greater portion of the year. The 

 depth of water appears to be the chief cause of a difference in the time of 

 spawning ; and it is exceedingly doubtful if on some deep beds they spawn 

 at all; and they are probably supplied by the fry drifting from some neigh- 

 bouring bed in shallower water. The commonly received opinion among the 

 fishermen, that the oyster deposits its spawn in masses, is entirely erroneous. 

 Oysters are best for the table out of shallow water, and at the entrance of a 

 river if suitable ground is found, and feed quicker in such situations. 



The author then read a series of questions, which it was requested any 

 person connected with oyster-beds would be kind enough to answer and 

 forward to him : — 1. Name of fishery ? 2. Depth of water? 3. Computed 

 size of beds ? 4. At what age do oysters spawn ? and do all oysters above 

 that age spawn ? 5. Does the time of spawning differ on different beds 

 within your knowledge ? 6. If such difference exist, is it caused by a 

 variation in the depth of water, or any other reason ? 7. What is the 

 ground ? 8. Do the oysters differ on different sorts of ground ? 9. Add 

 any other information. 



Report on Cleavage and Foliation in Rocks, and on the Theoretical 

 Explanations of these Ph(Bnomena. — Parti. .By John Phillips, 

 M.A., F.R.S., Reader in Geology in the University of Oxford. 



Of the numerous structures existing in rocks, two more predominant than 

 the rest have long been referred to their appropriate causes — sedimentary 

 deposition — crystalline aggregation. The 'strata,' formed by the first pro- 

 cess, have all the varieties of mineral substance and magnitude of grain, and 

 all the inequality of extent and bulk which we observe to occur in modern 

 deposits from water ; the granite and other quartzo-felspathic rocks offer a 

 large range of crystalline aggregates, always analogous to, and sometimes 

 undistinguishable from, the products of actual volcanoes. 



But in many, and especially in mountainous countries, examples occur of 

 rocks which seem both crystalline in texture and stratified in structure, and 

 others which are apparently formed by sediments, but are thoroughly fis- 

 sured to a degree of tenuity, and with a regularity and continuity not observed 

 in ordinary cases of stratification. The former case is exemplified in gneiss, 

 the latter in clay-slate. Giving to the divisions of gneiss the name of 

 ' foliation,' and to the fissures of slate the title of ' cleavage,' we may proceed 

 to trace the observations and inferences by which some light has been thrown 

 on these phsenomena. We begin with cleavage. 



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