In their description of isoglosses , they 
( In ( ( their description ) ( of ( ( isoglosses , ) they ) ) ) )
(ROOT (PP (IN In) (NP (NP (PRP$ their) (NN description)) (PP (IN of) (NP (NP (NNS isoglosses)) (, ,) (NP (PRP they)))))))
Isoglosses are difficult to understand.
( Isoglosses ( ( are ( difficult ( to understand ) ) ) . ) )
(ROOT (S (NP (NNS Isoglosses)) (VP (VBP are) (ADJP (JJ difficult) (S (VP (TO to) (VP (VB understand)))))) (. .)))