TRANSACTIONS OF SECTION C. 761 



making a firm 1)0110111 like cement. At Catworth, near Kimbolton, on the summit 

 of high ground overlooking the plain of the Oxford Clay, there is a mass of chalk 

 of great size, regularly interstratified with flint and lying on Boulder Clay. The 

 very unusual phenomenon is presented of a village, or the greater and principal 

 part of a village, built on chalk far avcay from any place where the Chalk forma- 

 tion occurs in place, or any outliers of that rock are seen. The evidence is striking. 

 There are ponds and pits about in bare chalk, the soil in the gardens is chalk, and 

 the graves in the churchyard leave off in chalk. There are numerous old excava- 

 tions besides, whence hundreds of loads of chalk have been got out and carted 

 away to the farms adjacent. 



The flints in this chalk are angular, and show little signs of being weathered or 

 worn, and there are in it besides thick tabular masses of flint. Copious springs 

 issue at the base of this chalk, and it is therefore an important water-bearing bed 

 in the village. The water in the wells sunk through the chalk to the clay beneath, 

 frequently runs over the top ; while in that portion of the village which is outside 

 the chalk area no water can be got by sinking in the clay. Besides Chalk there are 

 boulders of other rocks clustered about this village, but none of notable size. It is 

 not clear whether the Catworth ChaUc is all one boulder — it may, perhaps, be 

 several boulders with clay between — but as the material has been transported 

 unaltered from the parent rock, it is not, in any sense of the term, a reconstructed 

 chalk. 



15. Augen Structure in Relation to tJie Origin of Eruptive BocJcs and Gneiss, 



By 3. G. GooDCHiLD, F.0.8. 



The author discriminates between two types of augen structure — that (also 

 termed phacoidal structure) in which the ' eyes ' are not necessarily crystalline in 

 structure, but are the unsheared portions of the rock which have escaped conver- 

 sion into the schist to which their matrix has been reduced, and that in which the 

 ' eyes ' are crystalline minerals, generally undeformed, and of later date in origin 

 than the movements which have produced the schistosity. 



True augen structure occurs under two different conditions. In the one the 

 constituents out of which the augen have been formed were already in existence 

 within the rock, and their development in a crystalline form is merely a case of 

 regeneration under plutonic conditions. In the other class of augen structure 

 one or more of the essential constituents that go to form the ' eyes ' did not ori- 

 ginally exist within the rock, but have been introduced at a late period in its his- 

 tory from some foreign source. 



Both of these classes of augen structure appear to be due to segregatory action, 

 which came into play at a time when the rocks in which the structure occurs were 

 in a potentially molten condition arising from the heat developed by earth move- 

 ments acting under great pressure. Under these conditions of high temperature a 

 slight and very gradual relief of the pressure referred to permitted some of the 

 less refractory minerals to pass into a condition which favoured their segregating 

 from a state of diffusion throughout the mass. Under these circumstances the 

 more refractory minerals remained practically unaffected. If, following the dimi- 

 nution of pressure (which is equivalent in this case to a rise of temperature), there 

 ensued a fall of temperature, the newly-formed minerals passed into the crystalline 

 condition, while the rock material within which the augen had been developed 

 still retained the schistose or other structure impressed upon it by the earth move- 

 ments of prior date. 



According to this view, therefore, phacoidal amphibolite and augen-amphibolite 

 are respectively the results of mechanical and chemical action upon the same- 

 original type of rock. 



In the other types of augen structure the 'eyes' are developed by the 

 heat generated by earth movements, as in the former case; but an essential 

 component of one or more of the constituents of the augen has been derived 

 from an outside source. Augen of this class may consist of any one of several 

 minerals; but those of most importance in the present connection are the 



