434 Mr. Austen on the 



southern portion, but which includes perhaps a greater number of distinct forma- 

 tions ; presents them under circumstances more favourable for their study ; and 

 illustrates more points of geological interest than any other tract of equal extent in 

 England. The object of the memoir is to describe, 1st, The several sedimentary 

 formations ; their mutual relations, and organic remains. 2nd, The igneous rocks. 

 3rd, General conditions ; the amount of successive change and local disturbance. 



General Viev) of the Deposits of South Devon. 

 1. Actual Period. (Map, PL XLI.) 

 § 1. Estuary deposits. — Along the south coast of England, from the district where 

 the older rocks first show themselves, all the rivers pour their waters into salt or 

 brackish tidal estuaries, some of which are of considerable size ; and it is necessary 

 to notice them and certain points connected with such bodies of water, to enable 

 the reader to appreciate the evidence from which we infer that the estuaries were 

 at a former period more considerable, and that other valleys were once estuaries 

 .which have now ceased to be so. The rocks which bound the terminal valley of 

 the Teign are, in the lower part, sandstones and conglomerates of the new red 

 series ; and, in the upper, inferior rocks ; the length of the estuary being about 

 five miles, but its breadth when full is less than one. At low-water this space pre- 

 sents long banks of sand and of mud, black and fetid, from the abundance of de- 

 composing vegetable matter, but between the banks a few streams of fresh water 

 find their way : at high tide the whole area is submerged, and the water reaches the 

 base of a line of low cliffs on each side. Such also are the features of the estuary 

 of the Exe and of others to the westward. 



The species of mollusca living in these estuaries are few. The common Mytilus occurs only near 

 the openings with the sea. Cardium edule, Mactra compressa, Venus verrucosa and V. reflexa, ascend 

 much higher, and are very numerous ; and many curious instances may be observed of the power which 

 some marine animals acquire of adapting themselves to alternate change. When tides are low, and the 

 volume of the Teign considerable, the banks in which these moUusks live are covered by fresh water ; yet 

 the animals do not perish. A short distance above West Teignmouth are some sandstone beds, which 

 are tenanted by a colony of the Pholas dactylus, and from which remarkably fine specimens may be pro- 

 cured ; yet at low-tide the beds are covered by a stream of fresh water, which flows over them from the 

 land. In like manner, the Teredines, which entirely destroyed the piles of Teignmouth Bridge, were 

 constantly exposed to fresh water for several hours. But though the marine conchifera can thus accom- 

 modate themselves to change, the Uniones and other fluviatile shells never, that I am aware, descend into 

 the estuary ; so that there is always a considerable area which affords no testacea, a circumstance which 

 may account for the paucity of such remains in valleys now abandoned by the sea. 



§ 2. River sand and gravel. — The production of river sand and gravel, and the 

 forms of fluviatile pebbles as distinguishable from marine shingle, are facts famihar 

 to most observers. It is likewise well known that all streams which take their rise in 

 upland regions have their channels strewed with blocks ; yet the power of transport 



